1. Field of the Invention
The present invention generally relates to a method for producing a metal/ceramic bonding substrate. More specifically, the invention relates to a method for producing a metal/ceramic bonding substrate wherein a metal member is bonded to a ceramic substrate via an active metal containing brazing filler metal.
2. Description of the Prior Art
In a typical metal/ceramic bonding substrate used as an insulating substrate, cracks are easily produced in a ceramic member by a thermal stress based on the difference in thermal expansion which is caused between the ceramic member and a metal member by thermal shocks after bonding. As a method for relaxing such a thermal stress, there is known a method for thinning the peripheral portion (creeping portion) of a metal member, i.e. a method for forming a stepped structure or a fillet on the peripheral portion of a metal member.
In order to realize such a structure, there is known a method for forming a fillet on the peripheral portion of a metal circuit by steps of: printing a resist on a predetermined portion of a surface of a metal plate bonded to a ceramic member via an active metal containing brazing filler metal; etching unnecessary part of the metal plate to form metal circuits, and then, removing the resist; removing unnecessary part of the brazing filler metal remaining between the metal circuits; printing a resist on the surface of the metal circuits again to etch unnecessary part of the metal circuits again; and removing the resist, i.e., by carrying out a circuit pattern printing process and a metal circuit etching process twice, respectively.
There is also known a method for forming a ceramic circuit board having a metal circuit by removing unnecessary part of a brazing filler metal with a chemical containing an inorganic acid and hydrofluoric acid after a metal circuit pattern is formed on a ceramic substrate (see, e.g., Japanese Patent No. 2594475).
However, if the fillet is formed on the peripheral portion of the metal circuit by the method for carrying out the circuit pattern printing process and the metal circuit etching process twice, respectively, producing steps are complicated, and producing costs are increased by the increase of the number of steps.
In the method disclosed in Japanese Patent No. 2594475, since an etchant containing an inorganic acid and hydrofluoric acid is used for removing unnecessary part of an active metal containing brazing filler metal, there are some cases where a layer (which will be hereinafter referred to a “metal layer”), which is mainly formed of a metal other than an active metal of the active metal containing brazing filler metal, of two layers formed of the active metal containing brazing filler metal, i.e., the metal layer and a layer (which will be hereinafter referred to as an “active metal layer”) which is mainly formed of the active metal and its compound(s), may be excessively etched (side-etched), so that there are some cases where it is difficult to form the fillet. In addition, the etchant containing the inorganic acid and hydrofluoric acid is easy to damage ceramics, so that there are some cases where reliability, such as heat cycle resistance, may be deteriorated.